In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such process gas containing, among other components, carbon dioxide CO2. With increasing environmental demands various processes for removing carbon dioxide from the process gas have been developed.
CO2 capture often comprises cooling, or compression and cooling, of the flue gas to condense CO2 in liquid or solid form and separate it from non-condensable flue gas components, such as N2 and O2. Prior to CO2 capture, it is generally necessary to clean the carbon dioxide rich flue gas. Gas cleaning operation may generally include removal of dust, sulfur compounds, metals, nitrogen oxides, etc.
Cooling of the flue gas to its condensation temperature may be achieved by various means, e.g. using a suitable external refrigerant. CO2 capture systems using an external refrigerant can be expensive, both in terms of investment costs and in terms of operational costs. As an alternative, autorefrigeration systems are often used, wherein the CO2 rich flue gas is compressed, cooled and expanded to achieve condensation of the CO2. In these systems the liquid CO2 product is used as a cooling medium for the CO2 rich flue gas. Because of the tight temperature approaches between the condensing and evaporating media in these systems, CO2 condensation must generally be done using brazed aluminum heat exchangers. Besides being expensive, aluminum is sensitive to fouling by many of the trace components contained in flue gases from fossil fuel combustion like mercury and particulate matter. Autorefrigeration systems therefore generally require extensive efforts to remove harmful components in the flue gas upstream of the CO2 condensation step, such as particle filters, mercury adsorbers, and SOX/NOX scrubbers.